Method of making a dielectric core and resistor



J. A. ADAIR May 17, 1960 METHOD OF MAKING A DIELECTRIC CORE AND RESISTORFiled May 17. 1954 2 Sheets-Sheet l ELi l a. I

May 17, 1960 J. A. ADAIR 2,936,516

METHOD OF MAKING A DIELECTRIC CORE AND RESISTOR Filed May 17. 1954 2Sheets-Sheet 2 United States Patent METHOD OF MAKING A DIELECTRIC COREAND RESISTOR John A. Adair, Chicago, Ill.

Application May 17, 1954, Serial No. "430,375 13 Claims. (Cl.29--155.68)

This invention relates to dielectric cores, to electrical elementshaving dielectric cores for use in electrical instruments, and to amethod of making such cores and elements.

Dielectric cores for electrical elements have been made in several ways.In one method cores are formed from insulating fiberboard cut to desiredlengths from strips.

An operation is then required to smooth the sharp edges so that the wireto be wound later upon the core is not so apt to be cut. Other cores aremade from ceramic tubing. These methods are not adapted for use inmaking electrical elements in continuing strip form.

The primary object of the present invention is to provide a noveldielectric core of continuous length to be used in electrical elementsand having a substantially uniform cross-section.

Another object is to provide a novel method of forming such a dielectriccore of continuous length.

A further object is to provide a flattened dielectric core forelectrical elements which is slightly elastic when flexed axially in aplane normal to the plane of the flattened core.

Another object is to provide a novel method of forming annularly shapedwire-wound resistors adapted for use as potentiometers or rheostats inconnection with the manual controls of electrical circuits.

The invention is illustrated in a preferred embodiment in theaccompanying drawings, in which:

Fig. 1 is a diagrammatic view of the preferred method of formingdielectric cores in continuous strip form for use in making wire-woundelectrical elements;

Fig. 2, a broken perspective view of a portion of the fibrous materialstrip from which a dielectric core is made;

Fig. 3, a broken perspective view of a portion of the dielectric core onwhich several turns of a wire winding have been made to illustrate theform of the completed electrical element strip;

Fig. 4, a perspective view showing a cutter bar separating an electricalelement length from the completed electrical element strip;

Fig. 5, a sectional view taken as indicated on line 5-5 of Fig. 4;

Fig. 6, a broken perspective view showing the preferred method offorming the electrical strip into a helical coil by using a mandrel;

Fig. 7, a perspective view of a single portion of the helical coil shownin Fig. 6 with terminals fixed to its opposite ends; and

Fig. 8, a broken perspective view of an electrical element formed from alength as shown in Fig. 4, said element coated with an insulating cementand having terminals secured to its ends.

Referring to the drawings, and particularly to Fig. 1, a flexiblewoven'fibrous strip of material 10, usually :purchased in continuousstrip form upon a cylinder or spool 11, is pulled from the spool throughvarious process stations by a plurality of feed rollers. For clarity2,936,516 Patented May 17, 1960 2 only feed rollers 23 are shown. Thestrip 10 is treated with a stiffening agent and ultimately used as adielectric core 9 for an electrical element.

The material of the strip 10 is preferably made from glass fibers wovento form a sleeve structure, but it is not intended that the invention belimited to the use of this sleeve formation. A so-called cable weave ispreferred so that the sleeve can be stretched to a point where a uniformcross-section is attained, at which point additional tensioning forceswill not materially alter the crosssection throughout the length of thetensioning sleeve. With such a weave the tensioning force need not be soaccurately controlled; that is, a minimum tensioning force sufficient tofully stretch the sleeve can easily be maintained.

The strip 10 may first be passed through a radiant oven 13 to burn outorganic materials, mainly starches and oils, inherent in the manufactureof the woven glass sleeving. Although higher temperatures may be used,the temperature in the oven is maintained between 550 F. and 750 F., thespeed of the sleeve in passing through the oven 13 being varied indirect relation to the temperature. 7

During the application of the stiffening agent to the sleeve ofmaterial, and during subsequent drying and winding operations, both tobe discussed later, the strip 10 is preferably maintained under asubstantially uniform tensioning force, so that the cross-section of thestrip 10 will be substantially uniform throughout its length as it isformed into a dielectric core. This tension is maintained by a feedermechanism, generally designated 12, and a tensioning mechanism,generally designated 14.

As shown in Fig. l, the pulley 14a of the tensioning mechanism isnormally pivoted in a downward direction by a weight 15 suspended on alever arm 16. As the strip is pulled from the spool 11, some addedtension is exerted on it to overcome the friction and inertia of thespool. The added tension moves the pulley 14a upwardly, actuating amicro switch (not shown) which in turn actuates the feeder mechanism 12.After the feeder mechanism pulls an additional length of material fromthe spool 11, the pulley 14a moves downwardly under gravity, breakingcontact with the micro switch and turning off the feeder 12. In normaloperation this cycle is repeated quite rapidly. The resultingintermittent feed prevents gross movement of the tensioning mechanism,and a substantially uniform tension is maintained in the strip 10 at alltimes.

From the tensioning mechanism 14 the fibrous strip 10 is passedoveranother pulley 14b and then into a container 17 filled with a stiffeningagent 18. A pair of pulleys '19 and 20 guide the strip 10 between a pairof doctor blades 21 and 22 which are spring urged into contact withopposite faces of the strip 10 to wipe the excess solution. fromthestrip before it leaves the container 17. t

The stiffening agent preferred is a solution of an alkyl aryl silicone.However, dihydrogen aluminum phosphate, sodium silicate, or a mixture ofdihydrogen aluminum phosphate and aluminum phosphate have been found toserve as satisfactory stiffening agents. The purpose of the stiffeningagent is to lock the fibers of the strip 10 in their tensioned position.In addition, it imparts high temperature resistant qualities to the core9 and enablesthe core 9 to withstand temperatures in excess of thosefound in operating resistors, rheostats, potentiometers, etc.

over the surface 24. The oven 25 is generally maintained at temperaturesvarying from 250 F. to 500 F., depending upon the speed of the strip inpassing through it and the degree of cure desired in the particularstiffening agent used. Upon leaving the oven 25, the strip 10 is shapedlike a flattened sleeve having generally rounded marginal edge portionswith the opposite inner surfaces of the sleeve normally adhering to eachother to form a hardened laminate-like core. The strip 10 at this timeis slightly elastic and may be axially flexed in a plane normal to theplane of the flattened surfaces of the strip.

The strip 10 is then passed into a conventional wire winding mechanism,generally designated 26. This mechanism operates in timed relation tothe feed rollers 23 to determine the number of turns of wire per unitlength of the strip 10. In the drawings, resistance wire 26a is shownwound upon the stiffened strip 10 to form a continuous-1ength electricalelement strip 34. The turns of wire may be spaced to provide airinsulation between each turn, or the wire may be'procured with aninsulating coating. The carefully controlled cross-section of the strip10, together with the properly spaced windings about the core, producesa resistance strip having a uniform resistance per unit length.

In the drawings only resistances are shown, but core- .wound inductancescan also be formed by the method of this invention.

After the winding operation, the electrical resistance strip may bewound upon a cylinder, not shown, and removed to storage to awaitfurther use. However, if desired, the continuous strip 34 may beprocessed further to form individual electrical elements.

Fig. 6 illustrates one way in which the strip 34 may be prepared priorto being formed into a plurality of rounded resistance elements, such aselement 33 shown in Fig. 7. This element 33 is particularly well adaptedfor use in a rheostat which functions as a variable control inelectrical circuits. The strip 34 is passed from a winding mechanism 26through a tank 31 containing a bath of a softening agent 30. Thesoftening agent should be a plasticizer or solvent which will soften thestiffening agent and lend pliability to the core 9. In the preferedmethod, methylene dichloride which is non-inflammable is used for thesilicone-softening agent. It evaporates rapidly, and so may be readilyeliminated in the subsequent drying operation.

If the strip 34 is only partially cured in the radiant oven 25, furtherheating will initially render the core 9 of the strip 34 pliable withoutthe use of a softening agent. The softened core may then be shaped asdesired and subsequently be fully cured by further heating or merely becooled to a set in its new formation.

From the tank 31 the strip 34, still in its tensioned condition, istightly wound upon a mandrel 32 to form a helical coil 35.

When the mandrel is filled, the strip is cut, and the mandrel and thecoiled resistant strip are taken away and dried. The drying may be doneunder ordinary strip 34 being fed from the winding mechanism 26 intoacutter 27 which severs the strip into resistance elements 27a of thedesired lengths. Terminals may be fixed to opposite ends of the element27a to form a resistor immediately.

An element 27a may also be formed into a resistor of any shape desired.It is first immersed in a softening agent of the type previouslydescribed, and then bent to the desired form after which the softeningagent is removed by drying. Fig. 8 illustrates a U-shaped resistorhaving terminals 28 secured to its opposite ends. A coating 29 of aninsulating material is then applied to the preformed resistor.Preferably, but not by way of limitation, the resistor is coated bydipping or rolling it in a slip of insulating cement. The coatedresistor is then heated, preferably at about 550 F. for from thirty-fiveminutes to an hour, to harden the insulating cement and to curecompletely the stiffening agent in the core 9 at the same time.

It is also contemplated by this invention that the annular-type resistor33 may be provided with an insulating material embracing one marginaledge portion, leaving the opposite marginal edge portion exposed forcontact with a slidable element of the type used in volume controls forelectrical circuits. Such a coating may be formed from insulating cementapplied to each individual resistor 33. Preferably the coating isapplied to the continuous strip 34 by folding another strip, treatedlike the strip 10 with a stiffening agent 18, about one marginal edgeportion of the strip 34 as it leaves the winding mechanism 26. Asubsequent heat treatment will cause the folded strip to adhere to thestrip 34, and it then may be passed through the solution 30 and woundupon the mandrel 32. Individual elements similar to resistor 33 may thenbe prepared as previously described.

The foregoing detailed description is given for clearness ofunderstanding only and no unnecessary limitations should be understoodtherefrom, for some modifications will be obvious to those skilled inthe art.

I claim:

1. The method of forming a wire-wound electrical resistor, comprising:tensioning a length of fibrous material to obtain a substantiallyuniform cross section in said material throughout its length;impregnating said tensioned length of material with ahigh-temperature-resistant dielectric stiffening agent; drying saidimpregnated tensioned length so that said stiffening agent will lock thefibrous material of the length in said tensioned condition to form aresistor core; and uniformly winding a resistance wire of uniformresistance per unit length about said resistor core to form a length ofresistor having a uniform resistance per unit length.

2. The method of forming an electrical resistor, comprising: fonning aresistor core length from a tensioned woven fibrous material, the corelength being stiflened by a coating of a partially cured silicone resin;winding a. length of resistance wire about the stiffened core length toform a resistor element; heating said resistor element to soften thepartially cured silicone resin; bending the resistor element to form ahelical coil; and cooling said helical coil to harden the silicone resinand core length to form a helically-shaped electrical resistor.

3. The method of forming a plurality of substantially circularwire-wound electrical resistors, comprising: forming an electricalresistor strip having resistance wire coiled about a flat, laminate-likecore formed from a tensioned woven glass fiber sleeve, said core beingstill?- ened by a coating of a partially cured silicone resin; applyinga softening agent to said strip to render the core pliable; bending saidstrip to form a helical coil; drying said helical coil to evaporate saidsoftening agent and again lock the fibers of the core in position; andcutting said helical coil longitudinally to form a plurality ofring-shaped resistor elements.

4. The method of forming a plurality of substantially circularwire-wound electrical resistors, comprising: forming an electricalresistor strip having resistance wire coiled about a flat, laminate-likecore formed from a tensioned woven glass fiber sleeve, said core beingstiffened by a coating of a partially cured silicone resin; heating saidstrip to soften the partially cured silicone resin; bending said heatedstrip to form a helical coil; cooling said helical coil to harden saidsilicone resin so that said core is again stifiened; and cutting saidhelical coil longitudinally to form a plurality of ringshaped resistorelements.

5. The method of forming individual resistors from a wire-woundelectrical resistor strip, comprising: forming an electrical resistorstrip having resistance wire coiled about a flat, laminate-like coreformed from a tensioned woven glass fiber sleeve, said core beingstiffened by a coating of a partially cured silicone resin; cutting theelectrical resistor strip into the desired lengths; heating each of saidlengths to soften the partially cured silicone resin and render eachlength pliable; forming each length into the desired resistor shape; andheating each of said lengths again to cure the silicone resin fully sothat the silicone resin will lock the fibers of the core in the desiredshape.

6. The method of forming individual resistors from a wire-woundelectrical resistor strip, comprising: forming an electrical resistorstrip having resistance wire coiled about a fiat, laminate-like coreformed from a tensioned woven glass fiber sleeve, said core beingstiffened by a coating of a partially cured silicone resin; cutting theelectrical resistor strip into the desired lengths; applying a softeningagent to each of said lengths to render each length pliable; formingeach length into the desired resistor shape; and drying each of saidlengths to evaporate the softening agent and permit the silicone resinto lock the fibers of the core in said desired resistor shape.

7. The method of claim 6 having the added steps of securing electricalterminals to opposite ends of each of the shaped resistor lengths,dipping each of said resistors into a slip of insulating cement, andheating each of said electrical resistors to give a permanent set tosaid cement and to cure fully the stifiening agent at the same time.

8. The method of forming a core for an electrical element, comprising:tensioning a length of woven fibrous material to obtain a substantiallyuniform crosssection throughout said length; impregnating said tensionedlength of material with a high-temperature-resistant dielectricstiffening agent; and drying said impregnated tensioned length to permitsaid stiffening agent to lock the fibrous material of the length in saidtensioned condition.

9. The method of forming a core for an electrical element, comprising:tensioning a sleeve of flexible fibrous woven material to form asubstantially uniform cross section throughout the length of saidsleeve; impregnating said tensioned sleeve with ahigh-temperature-resistant dielectric stiffening agent; applyingpressure to the surface of the sleeve to fiatten said sleeve; and dryingsaid impregnated, tensioned sleeve to permit said stiffening agent tolock the fibrous woven material of the flattened sleeve in saidtensioned condition.

10. The method of claim 9 including the step of forcing the opposedinner surfaces of the flattened sleeve together during the dryingoperation so that said surfaces will adhere to each other when dry toform a laminatelike core.

11. The method of forming a core for electrical elements in a continuousstrip, comprising: tensioning the leading portions of an advancingsleeve of flexible fibrous material in continuous strip form as saidportions pass a predetermined point so that said leading portions have asubstantially uniform cross section; impregnating said tensioned leadingportions with a high-temperatureresistant dielectric stiffening agent;and drying said impregnated, tensioned leading portions so that saidstiffening agent will lock the fibrous material of the leading portionsin said tensioned condition.

12. The method of forming a core for an electrical element, comprising:passing a glass fiber woven sleeve through a first radiant oven to driveoff organic compounds present in the glass fiber; pulling the sleevetaut until it assumes a substantially uniform flattened cross sectionthroughout its length; passing the sleeve through a solution of astiffening agent to impregnate the fibers of said sleeve with ahigh-temperature-resistant dielectric material; removing the excess ofsaid stiffening agent from the exposed surface of said sleeve; andheating said sleeve to harden it into a laminate-like core, saidstiffening agent locking the woven glass fiber sleeve in its tensionedposition to maintain said substantially uniform cross-section.

13. The method of forming a core for an electrical element, comprising:passing a glass fiber woven sleeve through a first radiant oven to driveoff organic compounds present in the glass fiber; pulling the sleevetaut until it assumes a substantially uniform flattened crosssectionthroughout its length; passing the sleeve through a solution of astiffening agent to impregnate the fibers of said sleeve with ahigh-temperature-resistant dielectric material; removing the excess ofsaid stiffening agent from the opposite flattened surfaces of saidsleeve by passing it between a pair of opposed doctor blades; andheating said sleeve to harden it into a laminate-like core adapted toreceive windings of wire, said stiffening agent locking the woven glassfiber sleeve in its tensioned posi tion to maintain said substantiallyuniform cross-section.

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